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TRS-80 Model 4

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TRS-80 Model 4
Radio Shack TRS-80 Model 4 (Blake Patterson)
An early TRS-80 Model 4 with two disk drives (photo: Blake Patterson, CC BY 2.0)
Specifications
ManufacturerTandy Corporation, sold by Radio Shack
TypeDesktop computer
Released26 April 1983
Intro priceUS$999 (16 KB cassette), $1,699 (one drive), $1,999 (two drives)
CPUZilog Z80A, 2 or 4 MHz
Memory16 KB or 64 KB, expandable to 128 KB
StorageCassette; up to two internal and two external floppy drives
DisplayBuilt-in 12-inch monochrome monitor; 80 x 24 or 64 x 16 text
Sound—
ROM14 KB Model III BASIC
PowerInternal switching supply: 38 W (cassette model) or 65 W (disk models)
OS / FirmwareTRSDOS 6; Model III operating systems; CP/M
PredecessorTRS-80 Model III
SuccessorTRS-80 Model 4D
Model no.26-1067, 26-1068, 26-1069; 26-1069A (gate array)

The TRS-80 Model 4 is the successor to the TRS-80 Model III, introduced by Tandy's Radio Shack on 26 April 1983. It runs Model III software unchanged and adds a 4 MHz mode, an 80 x 24 screen, 64 KB of RAM that can be switched in over the ROM, an extra 64 KB bank and a larger keyboard with function keys.[1] The computer, keyboard, 12-inch monitor, power supply and up to two disk drives share one case, as on the Model III.[2]

History

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Radio Shack launched the Model 4 to the press on 27 April 1983 at an event run by the Boston Computer Society. It was sold in three versions:[1]

Model 4 versions at launch
Catalogue no. Configuration Price
26-1067 16 KB, cassette only $999
26-1068 64 KB, one floppy drive $1,699
26-1069 64 KB, two floppy drives $1,999

It shipped with TRSDOS 6, written for Radio Shack by Logical Systems, and can also run CP/M. By 1985 the two disk models had fallen to $1,099 and $1,299. Early main boards had a socket for Zilog's Z800, a planned 16-bit successor to the Z80 that Zilog never produced; later boards dropped it. Reed records the Model 4 as Radio Shack's best-selling computer until 1986.[1]

From late 1984 Tandy built a redesigned version, catalogue number 26-1069A, known as the gate array Model 4. Its main board uses gate arrays in place of the original logic and PALs, and carries the floppy controller and RS-232 circuits that the original machine had on separate boards. Gate array machines have a green screen, the four arrow keys grouped together, and an RS-232 connector facing the rear.[3] The TRS-80 Model 4D of 1985 is the gate array Model 4 with double-sided drives.[4]

Specifications

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TRS-80 Model 4 (original version)
Item Detail
CPU Z80A, 4 MHz, or 2 MHz in Model III mode
ROM 14 KB, Model III BASIC
RAM 16 KB (cassette model) or 64 KB; 128 KB with a second bank of 4164s and a PAL at U72
Display Built-in 12-inch monitor, RCA or TCE; 64 x 16 and 32 x 16 in Model III mode, 80 x 24 and 40 x 24 in Model 4 mode
Storage Cassette at 500 or 1500 baud; two internal drives on disk models; two more external
Boards CPU board; floppy disk controller board and RS-232C board on disk models
Power supply 38 W Astec AA11320 (cassette model); 65 W Astec AA12090 or Tandy 8790049 (disk models)

Sources: Tandy's technical reference manuals.[2][5][6]

Hardware

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Original version

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Radio Shack TRS-80 Model 4 CPU board 8709296 (Shelby Jueden)
CPU board of an original Model 4: bare board 8709296, here revision C. The Z80A is at U57 and the three ROMs at U68-U70 (photo: Shelby Jueden, CC BY-SA 4.0).

The CPU board stands at the back of the machine behind an RFI shield. Tandy used two assemblies: 6700104AA3 for the 16 KB cassette model and 8858090 for the 64 KB disk models, both on bare board 8709296. The board carries the Z80A (U57), the Model III BASIC ROMs (U68-U70), the PALs that handle memory mapping, the video circuit and up to sixteen 4164 RAMs.[7] Moving from 16 KB to 64 KB means fitting eight 4164s at U77-U84, moving three jumpers and removing eight 0.1 µF capacitors; going to 128 KB means adding eight more 4164s at U85-U92 and a PAL (Tandy 8075468) in place of the shunt at U72.[5]

The floppy controller and RS-232C interface are on separate boards bolted behind the CPU board. Two controller boards were used, both built round an FD1793: 8858060, whose data separator and write precompensation are set with three potentiometers, and 8858160, which uses a 9216 data separator chip.[8] The internal drives in the manual are Tandy part 8790112, made by TPI.[9]

Power supplies

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Tandy fitted three supplies depending on the configuration:[5]

  • 8790021, a 38 W Astec AA11320, in the 16 KB cassette model, mounted on the CPU bracket. If drives were added later, a second supply went on the left drive bracket.
  • 8790043, a 65 W Astec AA12090, or 8790049, Tandy's own 65 W design, in the disk models, mounted on the left side of the left drive bracket.

All three are switching supplies running from the rectified mains. Sams' service data for the gate array Model 4 has a schematic for the main supply and lists the two Astec units as alternates.[10]

Monitor

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The 12-inch monitor runs from +12 V. The manual covers two makes: RCA (Tandy 8492002, RCA KTR131B and C), with a P4 white phosphor tube, and TCE (8790607).[11]

Gate array version

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On the gate array board, gate array 4.1 (U9) generates the system clocks from a 20.2752 MHz crystal, a PLL module (U8) makes the 12.672 MHz clock for 80-column video, and the WD1773 floppy controller and the serial UART are on the main board.[12][10]

Known faults

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  • The mains filter capacitors in the supplies crack with age and then fail with a pop and smoke at switch-on. On the Astec AA12090 these are the three metallised-paper capacitors C1, C2 and C13.[13][14][15]
  • Garbage on the screen at switch-on that clears with RESET points to the power-on reset capacitor, C20 on the original board.[14]
  • An original board that shows a scrambled screen when TRSDOS 6 starts may need its trimmer capacitor near the floppy cable connector turned slightly; it sets the horizontal sync of the 80-column mode.[14]

See the TRS-80 Model 4 Troubleshooting Guide and the TRS-80 Model 4 Capacitor Replacement Guide.

Documentation

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  • Radio Shack TRS-80 Model 4 Technical Reference Manual: Tandy's 1983 manual for the original Model 4 (26-2110), with parts lists for every board, the three power supplies and both monitors, and the hardware part of the 1985 Model 4/4P manual (26-2119), which adds the gate array theory of operation.
  • Sams Computerfacts CS13 for the gate array Model 4 has schematics, photographs and troubleshooting procedures; a scan is on archive.org.
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References

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  1. ↑ 1.0 1.1 1.2 Matthew Reed, "The TRS-80 Model 4", TRS-80.org (retrieved 2026-10-21). Introduction on 26 April 1983, configurations, catalogue numbers and prices, Model III compatibility, new features, Z800 socket, TRSDOS 6.
  2. ↑ 2.0 2.1 Radio Shack, TRS-80 Model 4 Technical Reference Manual, catalogue no. 26-2110, 1983, hosted as File:Radio Shack TRS-80 Model 4 Technical Reference Manual (26-2110, 1983).pdf. Section I, "System Overview", p. 3.
  3. ↑ "TRS-80 Model 4", Wikipedia (retrieved 2026-10-21). Section "Gate Array Model 4": catalogue number 26-1069A, gate arrays in place of PALs, FDC and RS-232 on the main board, green screen, grouped arrow keys, rear-facing RS-232 connector.
  4. ↑ Tandy Corporation, Addendum to the Model 4 and Model 4 Gate Array Service Manual (Cat. No. 26-1067/8/9) for Model 4D (Cat. No. 26-1070), 1985, archive.org scan. Introduction, p. 1.
  5. ↑ 5.0 5.1 5.2 TRS-80 Model 4 Technical Reference Manual (26-2110, 1983), 1.3 "Jumper Options", p. 5; 2.1.5 "Main Power Supplies", p. 9; Sections III, IV, VI, VII and IX (CPU, FDC, power supply, video monitor and RS-232C boards).
  6. ↑ Tandy Corporation, TRS-80 Model 4/4P Technical Reference Manual, catalogue no. 26-2119, hardware part, 1985, hosted as File:Radio Shack TRS-80 Model 4 and 4P Technical Reference Manual, hardware (26-2119, 1985).pdf. 1.1.1 "Introduction", p. 3: 4 MHz Z80, 14K BASIC ROM, 64 x 16 display, up to 128K RAM, 500 or 1500 baud cassette.
  7. ↑ TRS-80 Model 4 Technical Reference Manual (26-2110, 1983), 2.1.2 "CPU Board", p. 9, and parts lists for CPU PCB 6700104AA3 (pp. 49-52) and CPU PCB 8858090 (pp. 53-56).
  8. ↑ TRS-80 Model 4 Technical Reference Manual (26-2110, 1983), Section IV "Floppy Disk Interface": 4.1 FDC PCB 8858060 (parts list p. 67) and 4.2 FDC PCB 8858160 (parts list p. 78).
  9. ↑ TRS-80 Model 4 Technical Reference Manual (26-2110, 1983), Section V "Mini-Disk Drive", 5.1 "Mini-Disk Drive #8790112 (TPI)", p. 81.
  10. ↑ 10.0 10.1 Howard W. Sams & Co., Computerfacts: Radio Shack TRS-80 Model 4 Gate Array Version, CS13, 1986, archive.org scan. Power supply schematics and the parts list, pp. 29-30, which name power supplies 8790021 (Astec AA11320) and 8790043 (Astec AA12090) as alternates.
  11. ↑ TRS-80 Model 4 Technical Reference Manual (26-2110, 1983), Section VII "Video Monitor": 7.1 RCA 8492002, specifications and service adjustments, pp. 145-146; parts lists pp. 150-154; 7.2 TCE 8790607, pp. 155-160.
  12. ↑ TRS-80 Model 4/4P Technical Reference Manual (26-2119, 1985), Section II, "Model 4 Gate Array Theory of Operation", 2.1.1-2.1.4, pp. 21-28.
  13. ↑ TRS-80 Model 4 Technical Reference Manual (26-2110, 1983), parts list for power supply 8790043 (Astec AA12090), p. 118.
  14. ↑ 14.0 14.1 14.2 Ira Goldklang, "TRS-80 Troubleshooting Guide and Repair Q&A", Ira Goldklang's TRS-80 Revived Site (retrieved 2026-10-21). Community repair answers.
  15. ↑ Michael Lohmeyer, "Fixing a Blown TRS-80 Model III/4 Power Supply", Ira Goldklang's TRS-80 Revived Site, 2013 (retrieved 2026-10-21).


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